// @FileName  :points.c
// @Time      :2024/2/29 22:11
// @Author    :YKW
//# include <bits/stdc++.h>
//using namespace std;
# include <stdio.h>
# include <stdlib.h>
# include <math.h>
# include <string.h>
# include <stdlib.h>

typedef struct po {
    int x;
    int y;
} po;

double len(po a, po b) {
    return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}

double calc(po a, po b, po c) {
    double l1 = len(a, b), l2 = len(a, c), l3 = len(b, c);
    double p = (l1 + l2 + l3) / 2;
    return sqrt(p * (p - l1) * (p - l2) * (p - l3));
}

void solve() {
    int n;
    po li[20];
    double ans = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
        scanf("%d%d", &li[i].x, &li[i].y);
    for (int i = 2; i < n; i++)
        ans += calc(li[0], li[i - 1], li[i]);
    printf("%.2lf", ans);
}

void main() {
    int t = 1;
    //scanf("%d", &t);
    while (t--) solve();
    return 0;
}